1. Field
The present disclosure relates generally to electronics, and more specifically, to an active-RC filter with compensation to reduce Q-enhancement.
2. Background
Filters are used extensively today in almost every electronic application. A filter is an electrical network that alters the amplitude and/or phase characteristics of a signal as the frequency of the signal changes. Thus, a filter may be used in an electronic circuit to pass signals in certain frequency ranges and attenuate signals in other frequency ranges. The behavior of a filter may be described mathematically in the frequency-domain in terms of its transfer function. The transfer function describes, among other things, the ratio between the input signal amplitude and the output signal amplitude applied to the filter. The amplitude response curve describes the effect of the filter on the amplitude of the input signal at various frequencies. The steepness of the amplitude response curve is generally described in terms of the filter's quality factor
Active filters may use amplifying elements, such as operational amplifiers (“op-amps”), with resistive and capacitive feedback, to generate the desired filter response. Active filters can be used to eliminate the need for inductors. These low-pass filters may be designed with a transfer function that has a steep rolloff around the pole frequency due primarily to the quality factor Q of each amplifying element. Unfortunately, the performance of these filters may suffer at high frequencies. In particular, the limited gain-bandwidth product of each amplifying element may cause Q-enhancement. Q-enhancement typically shows up as an undesirable peak in the amplitude around the pole frequency. This effect may be further exacerbated by the finite output impedance of the amplifying elements.
One way to reduce Q-enhancement is to design amplifying elements with a slightly lower Q, such that after Q-enhancement the desired Q-value results. One technique is to use “pre-distortion.” The problem with using pre-distortion is that it is difficult to achieve low passband ripple across process, voltage and temperature variations.
Another way to reduce Q-enhancement is to construct an amplifier with a final output buffer stage using a source follower resulting in low output impedance. Unfortunately, the use of such a buffer stage may degrade performance, for example by limiting the output voltage swing of the operational amplifier and increasing the DC current of the operational amplifier.
Accordingly, there is a need in the art for high frequency, high-Q filters with low Q-enhancement across process, voltage and temperature variations using amplifying elements without a final output buffer stage.